PFCs are used in many manufacturing processes. In particular, they are widely used in the manufacture of semiconductor components. The nature of many of these manufacturing processes results in the atmospheric emission of PFCs. Being of high value and of detriment to the environment, it is advantageous to recover these emitted PFCs so that they may be reused.
Examples of PFCs are nitrogen trifluoride (NF.sub.3), tetrafluoromethane (CF.sub.4), trifluoromethane (CHF.sub.3), hexafluoroethane (C.sub.2 F.sub.6) and sulfur hexafluoride (SF.sub.6). In general, PFCs are fully fluorinated compounds of nitrogen, carbon and sulfur. CHF.sub.3 is an example which is not fully fluorinated, but due to its similar chemical nature and application with other fluorine saturated PFCs, it is considered a PFC.
The manufacture of semiconductor components produces exhausts which typically comprise PFCs, non-PFC gases, particulate matter and a carrier gas. The flow from one process tool may be as high as 400 standard cubic feet per hour (scfh) and may comprise less than 1% PFCs.
Non-PFC gases may include hydrogen fluoride (HF), silicone tetrafluoride (SiF.sub.4), silane tetrahydride (SiH.sub.4), carbonyl fluoride (COF.sub.2), carbon dioxide (CO.sub.2), water (H.sub.2), methane (CH.sub.4) and carbon monoxide (CO). The carrier gas may be air, nitrogen or another inert gas. The majority of non-PFC gases and particulates are detrimental to PFC recovery processes and need to be removed in pre-purification processes. Some of the non-PFC gases, for example, carbon monoxide, may be inert to PFC recovery processes and may be allowed to pass through with the carrier gas.
The present invention recovers PFCs from the pre-purified carrier gas by condensation utilizing the large differences between the boiling points of PFCs and of various carrier gases. Table 1 gives the atmospheric boiling points and melting points of some common PFCs and nitrogen.
TABLE 1 Boiling Melting Compound Point (K.) Point (K.) N.sub.2 77 63 NF.sub.3 144 66 CF.sub.4 145 90 CHF.sub.3 191 118 C.sub.2 F.sub.6 195 173 SF.sub.6 209 222
Condensation of the PFCs is achieved by cooling the gas stream to temperatures below the dew points of the constituent PFCs. In order to achieve a high PFC recovery efficiency, it is necessary to cool the gas stream below the melting points of some of the lower volatility PFCs. Freezing of PFCs in the condenser is undesirable since this would reduce the efficiency of the condenser and prevent continuous operation. The instant invention contains several facets which prevent PFC freezing.
One, a reflux condenser is preferably used to effect the condensation of PFCs. The condensate in a reflux condenser flows counter-currently to the gas flow from where it came, and is therefore not necessarily further cooled. In general, however, a conventional condenser is applicable in this invention.
Two, this flow regime means that high volatility PFC condensate flows over the regions where low volatility PFCs condense. Low volatility PFCs with a tendency to freeze are soluble in these high volatility PFCs and freezing can be prevented.
Three, in the preferred embodiment, the concentration of high volatility PFCs in the gas stream is raised by recycling them in the system. This is achieved by separating the high volatility PFCs from the recovered PFC product, followed by re-addition up-stream. Raising the concentration of high volatility PFCs in the gas stream lowers the concentration of low volatility PFCs in the PFC condensate and prevents the PFCs from freezing.
Various solutions to recover PFCs from a carrier gas stream has been suggested, some mitigating the problem of PFC freezing when cryogenic means is used for recovery. However, none of the art teaches or suggests the present invention.
A prior method for recovering PFCs from the carrier gas is by condensation/dissolution as shown in U.S. Pat. No. 5,626,023. A solvent is added to the gas stream, which is then cooled to condense out the PFCs and any vaporized solvent. Low volatility PFCs with a tendency to freeze are soluble in the additive solvent. The additive solvent and PFCs are then separated by distillation and the additive solvent is reused. The solvent must be completely removed from the PFC product to prevent loss.
U.S. Pat. No. 5,540,057 provides for the removal of volatile organic compounds (VOCs) from a carrier gas by condensation of the VOCs in a reflux condenser. The VOC laden carrier gas passes up the shell side of a shell and a tube heat exchanger, and is then cooled along a continuous temperature gradient. The VOCs condense out to different extents at different levels and collect on special baffles in the shell side, which can direct a portion out of the condenser and allow a portion to drip back down the condenser as reflux. The cold cleaned carrier gas is then mixed with refrigerant at the exit to the shell side and passes down the tube side to effect the shell side cooling. Freezing of VOCs, specifically benzene, may be inhibited by the addition of a solvent, specifically toluene, to the gas stream.
U.S. Pat. Nos. 5,533,338 and 5,799,509 are examples of condensation freezing for condensing PFCs against a cryogenic fluid. The freezing of low volatility PFCs occurs due to the low temperatures required for high efficiency condensation of the high volatility PFCs. This method is disadvantageous because it is necessary to periodically defrost the frozen PFCs for removal. This results in low refrigeration efficiencies and requires duplicate equipment in order to maintain continuous operation.
Membrane permeation recovers the PFCs from the carrier gas through the differences in membrane permeability. The gas stream is contacted with the feed side of a specific membrane, which allows the carrier gas to preferentially permeate while the PFCs are retained. High separation efficiencies require the use of multiple membranes. PFCs have different permeation characteristics and vary in recovery efficiencies.
Adsorption recovers PFCs from the carrier gas. The gas stream is contacted with an adsorbent which removes the PFCs. The PFCs are then desorbed and removed from the adsorbent bed with a sweep gas. The sweep gas results in a low concentration PFC product. Furthermore, adsorption processes do not have the flexibility to adjust to the large changes in PFC concentrations and carrier gas flow rates which typify gaseous effluent streams.
Yet another PFC recycling method is the energy intensive process of incineration. The gas stream is heated to a high temperature, which prevents emission of the PFCs. Decomposition gases such as hydrogen fluoride and nitrogen oxides are then removed from the flue gas.
It is desirable that PFC recovery systems treat the exhaust from small semiconductor manufacturing tool clusters rather than whole manufacturing facilities. If one system fails, only a fraction of the manufacturing tools are affected. The present invention is therefore primarily intended to treat the exhaust from a small number of tools. However, it may also be scaled-up to treat the exhaust from an entire semiconductor manufacturing facility. It is also an object of this invention to mitigate the problem associated with PFC freezing, while recovering them from a carrier gas stream by cryogenic condensation.